Accessing type annotations via annotation processor

Joe Darcy joe.darcy at
Tue Aug 19 00:40:13 UTC 2014


On 08/15/2014 03:53 AM, Gunnar Morling wrote:
> Hi,
> 2014- 8-14 19:25 GMT+02:00 Joe Darcy <joe.darcy at 
> <mailto:joe.darcy at>>:
>     Hello,
>     On 08/13/2014 11:56 PM, Gunnar Morling wrote:
>>     Hi Alex,
>>     2014-08-11 20:13 GMT+02:00 Alex Buckley <alex.buckley at
>>     <mailto:alex.buckley at>>:
>>         There are some known problems with retrieving type
>>         annotations on type parameter declarations, in both the Core
>>         Reflection API and the Language Model API. I believe type
>>         annotations on types used in member declarations work OK. If
>>         you have the VariableElement for 'values', you can get the
>>         TypeMirror for 'List<@NotNull String>', then visit it as a
>>         DeclaredType and call getTypeArguments() to get a TypeMirror
>>         for '@NotNull String'. TypeMirror is an AnnotatedConstruct so
>>         it can return the annotations on the use of 'String'.
>>     I tried what you suggested, but I only obtained a TypeMirror for
>>     'List<String>' from the VariableElement, thus the TypeMirror for
>>     the type argument represents 'String' rather than '@NotNull
>>     String'. Is there any example or test in OpenJDK which shows the
>>     retrieval of type annotations via the Language Model API?
>     Since the AnnotatedConstruct interface is now implemented by both
>     elements and type mirrors, is *should* work go call the annotation
>     retrieval methods on a TypeMirror.
>>     Another issue is that the process() method of the annotation
>>     processor is never invoked for the @NotNull annotation if it is
>>     only used as type annotation (the processor will be invoked
>>     though if @NotNull is e.g. used as a traditional field-level
>>     annotation). Is this an expected behavior?
>     From the revised discovery process documented in
>     javax.annotation.processing.Processor:
>     "Annotations on  type uses, as opposed to annotations on elements,
>     are ignored when computing whether or not an annotation type is
>     present."
> Ah, I see. Thanks for the pointer.
> Are there any plans to remove this restriction?

No; that was a conscious design decision. Type annotation are relatively 
uncommon and we thought it would be a poor trade-off to require much 
more extensive scanning of sources to look for annotations in those 

> Our annotation processor for Bean Validation aims at detecting 
> constraint declaration errors such as "List<@Email Integer> email" 
> (applying the @Email constraint to Integers does not make sense), but 
> as it stands its not possible to implement such check for type 
> annotations atm.

To implement this functional, you could declare an annotation processor 
to process "*" and then do your own scans for the annotations of interest.


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